How do lithium-ion batteries get old?

1 Factors affecting aging The best operating temperature of power lithium batteries is 15°C-35°C, but in daily applications, it is impossible to fully meet the requirements of the battery. Therefore, the most common scenarios that affect battery aging are high temperature and low temperature. In addition to the environment, the battery operating parameters also have the purpose of accelerating or decelerating aging, so the selection of battery charging and discharging parameters has a significant impact. Under the external factors listed above, in the electrochemical reaction process of battery electrode materials, side reactions other than normal charge and discharge occur, leading to the occurrence of aging. 2 Typical aging process During the aging process, the specific details of the process are closely related to the choice of positive and negative materials, electrolytes and separators. This article explains the rough aging process and does not explain the specific materials in detail for the time being. 2.1 High temperature aging of 50℃ to 60℃ is the upper limit of the allowable operating temperature range for general lithium-ion batteries. The electrochemical reaction is carried out at a higher temperature, the electrolyte is active, and decomposition reaction is prone to occur. The decomposition product combines with the positive electrode material, which is the consumption of the positive electrode material; the positive electrode structure material is corroded, and the lattice structure is lack of sufficient materials. The support collapses, the vacancy of lithium ions is reduced, and the capacity of the positive electrode to accommodate lithium ions is reduced, causing the loss of battery capacity; at the same time, the product reflected by the positive electrode material wanders in the electrolyte and may adhere to the surface of the positive and negative electrodes. The surface of the electrode is covered with substances that cannot participate in the charge and discharge process, which hinders the smooth occurrence of the electrochemical process and increases the internal resistance of the cell. The influence of high temperature process on aging mainly occurs in the positive electrode, and the influence on the negative electrode is relatively small. 2.2 Low-temperature aging When the ambient temperature reaches below 0°C, the performance of the lithium-ion battery begins to be significantly affected by the low temperature. The SIE film is a passivation film formed by the reaction between the negative electrode material and the electrolyte during the formation of the cell, and has a protective function for the negative electrode material. During low-temperature operation, the SEI film grows and consumes part of the active lithium ions in the electrolyte, so that the concentration of conductive ions in the electrolyte is reduced, and the available capacity of the battery is permanently lost. The thickening of the SEI film makes it more difficult for lithium ions to pass through the film to reach the negative electrode. This is combined with the reduction of the concentration of conductive lithium ions, and the internal resistance of the cell increases accordingly. When charging at low temperature, especially when the charging current is relatively large, another side reaction of lithium elemental precipitation will occur in the negative electrode. At low temperatures, the activity of lithium ions decreases and it is barely charged, causing excess lithium ions to gather around the negative electrode. Before they can penetrate the SEI film to reach the negative electrode, they are deposited on the surface of the negative electrode to form a pure lithium layer. This process is easy to happen during charging at too low temperature and is irreversible. As the cycle of use accumulates, the elemental lithium will continue to accumulate, and dendrites continue to grow, making the risk of piercing the diaphragm continue to accumulate. Lithium-ion batteries work at low temperatures, and the aging problem mainly occurs in the negative electrode, and the side reaction of the positive electrode also exists, but the impact is not significant. 2.3 High-current charging and discharging discharge with a current exceeding the designed discharge capacity. On the one hand, the thermal effect of the current causes the battery's own temperature to rise, and the side reaction of high-temperature aging gradually intensifies; on the other hand, the large current brings excessive lithium ions To insert the positive electrode material, it will impact the stability of the material. High-current discharge also has problems with heat generation and the stability of positive electrode material deintercalation. At the same time, too much lithium ions are transported to the negative electrode, exceeding the capacity of the negative electrode, causing the deposition of elemental lithium to occur. Not only will the capacity be lost, but the risk of thermal runaway from long-term use will increase and the harm will be more serious. 2.4 Over-voltage and under-voltage charging and discharging Over-voltage charging and under-voltage discharge will bring about phase change problems of the cathode material, which reduces the vacancy of lithium ions and affects the maximum usable capacity of the battery. 2.5 Self-discharge The self-discharge of the battery cell occurs anytime and anywhere. When the temperature is higher and the amount of charge is higher, the self-discharge process is more significant. The self-discharge process will bring a common loss of reversible capacity and irreversible capacity of the battery. The self-discharge product adheres to the surface of the electrode, blocking the lithium ion channel, reducing the position of lithium ion insertion, and then causing the permanent capacity loss of the battery. Three-module aging lithium-ion batteries are connected in series and in parallel to form modules, and the aging of the modules is directly affected by the aging of single cells. In addition, the aging of the battery cell will bring about the deterioration of the consistency between the battery cells, so that the aging of the module will be amplified on the basis of the aging of the battery core. In addition to being affected by the aging of the battery cell, the module will also deepen the aging degree due to factors such as vibration and the oxidation and corrosion of conductive parts. The inner battery of the module and the bus bar, and the copper bar and the module terminal are in close contact by welding or screw connection to ensure that the resistance is within a reasonable range. The increase in connection resistance caused by vibration and oxidation has changed the distribution of the internal resistance of the module. These changes may affect the detection result of the cell voltage, and then affect the charging and discharging and equalization process of the cell. The harsh environment and overloaded working parameters make the aging process more significant and easier for researchers to observe. In fact, the aging process has been going on silently. The battery core has two lifespans, calendar life and cycle life. From the name of the calendar life, you can feel that aging does not sleep endlessly. Therefore, the research on the influence factors of battery cell aging is only trying to reduce the accelerated aging caused by illegal operation. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete.